1. Technical Field
The present invention relates to poppet exhaust valves for internal combustion engines, and more particularly to a cold or warm heading process and improved austenitic stainless steel alloys for making such valves.
2. Description of the Prior Art
The manufacture of poppet exhaust valves for internal combustion utility, automotive and truck engines conventionally is a multi-step process. A bar stock of predetermined diameter is provided. The bar stock is a stainless steel alloy. A blank of desired length is cut from the bar stock. The blank is then reduced in diameter, for instance by extrusion, for its length, except at one end. The head end of the blank, which has not been extruded is then coined to a larger cross-section. The blank is then heat treated and machined to the valve finished dimensions.
Most poppet exhaust valves for internal combustion utility, automotive and truck engines are hot forged. The shaping steps, including extrusion and coining, are normally performed in the temperature range of 2,000.degree. F. to 2,200.degree. F. Hot forging has been necessary because of the relatively large size of the blanks used for the engine valves, and the high temperature operating properties desirable in an exhaust valve. The large size of the blanks means that more metal has to be moved a greater distance in the forming steps. This requires the use of high forming temperatures. The requirement of good high temperature operating properties has heretofore required the use of compositions capable of forming only by hot-forging.
Most hot-forged valves are made of an austenitic chromium, manganese, nickel, stainless steel alloy having a high weight content of interstitial elements, e.g., carbon and nitrogen. The following Table 1 lists two alloys used to produce a large majority of the poppet exhaust valves for internal combustion engines.
TABLE 1 ______________________________________ Hot Forged Austenitic Exhaust Valve Materials Element 21-4N 21-2N ______________________________________ Chromium 20-22 19.25-21.5 Nickel 3.25-4.5 1.5-2.75 Manganese 8-10 7.00-9.50 Carbon 0.475-0.575 .50-.60 Nitrogen 0.38-0.50 .29-.40 Silicon 0.25 Max 0.25 Max Sulfur 0.06 Max 0.06 Max Phosphorus 0.05 Max 0.05 Max ______________________________________
The high interstitial content (C+N) of these alloys imparts high strength and deformation resistance to the valves throughout the valve utilization range of below room temperature to 1,600.degree. F. This is excellent for performance characteristics of the valves in service, but restricts the manufacturing options available to the manufacturer of the valves.
For instance, attempts at economically producing valves at lower temperatures, for instance room temperature, from these materials have not been successful. Parts will not form properly and tool life is unacceptable Mechanical crank and screw presses utilizing hot work tooling are used to produce these parts. The production rate is about 14 to 20 pieces per minute. This is a low production rate.
Other disadvantages of the state-of-the-art are poor tool life, distorted parts, excessive stock, and the added cost associated with straightening the valves and grinding excess material from the valves.
Another manufacturing process used to produce some utility exhaust valves is by cold header forming. The cold forming process differs from conventional hot forming. In this process coil stock of a predetermined diameter is provided in the annealed and coated condition. A blank of desired length is cut from the coil stock. On end of the blank is stamped to form a cone-shaped reduction or taper. This step is called "nosing". The blank is then reduced in diameter, for instance by extrusion, for its length, at the tapered end. The head end of the blank, which is the end opposite the nose end, is then upset to form a preliminary head. The preliminary head has a cross section smaller than the cross section of the finished head of the valve. The preliminary head is then coined to a larger cross section. The valve is then machined to its finished dimensions.
The cold header forming process provides a number of advantages compared to the hot forging process. This process results in much higher production rates, for instance 60-100 pieces per minute, and straighter more net shape parts. Also the cold forming work hardens the valves. This provides the valves, particularly the valve stem, with improved wear resistance and strength.
Disadvantages of cold heading are that only a few materials can be successfully processed into valve form and furthermore the size of these valves is severely limited. The following Table II gives the composition of one stainless steel alloy heretofore used for the manufacture of exhaust valves by cold heading.
TABLE 2 ______________________________________ Cold Headable Austenitic Exhaust Valve Material Element 302HQ ______________________________________ Chromium 17-19 Nickel 8-10 Copper 3-4 Manganese 2 Max. Silicon 1 Max. Carbon 0.08 Max. Sulfur 0.03 Phosphorus 0.045 ______________________________________
The alloy of Table II is characterized by the presence of low amounts of the interstitial elements carbon and nitrogen. Carbon is specified at a low maximum value and nitrogen is limited to its natural absorption from surface air contact. As a result the 302HQ alloy is readily cold formed. However, even with the highly formable alloy 302HQ, the extrusion and coining strains are severely limited. In terms of true strain, extrusion is limited to a strain of about one. Above this value, tool life deteriorates precipitously The limit for true strain, in coining the alloy of Table II, is about 1.8. Above this level part quality deteriorates. For instance, head splitting can result.
Therefore, this alloy is suitable only for low duty applications, such as small utility engines having poppet valves of relatively small size. This alloy also lacks the thermal stability and elevated temperature properties required for most internal combustion engine poppet exhaust valves.